The invention relates to aqueous 2-component polyurethane (PU) coating compositions containing specially modified vinyl polymer polyol dispersions and polyisocyanates, a process for the preparation of such aqueous 2-component PU systems with improved stability in respect of aqueous chemicals, and their use as 2-component PU paints for painting any type of substrate, e.g. wood, metal, plastic.
EP-A 0 358 979, EP-A 0 496 210 and EP-A 0 557 844 and other patent literature describe the preparation of aqueous 2-component PU dispersions in which both so-called secondary dispersions and so-called primary dispersions as polyol components are used with suitable polyisocyanates. The term xe2x80x9csecondary dispersionsxe2x80x9d refers to such aqueous dispersions which are first polymerized in the homogeneous organic medium and then redispersed in the aqueous medium with neutralization, generally without the addition of external emulsifiers.
The term xe2x80x9cprimary dispersionsxe2x80x9d refers to polyol dispersions that are prepared directly in the aqueous phase by the method of emulsion polymerization. They generally contain external emulsifiers for electrostatic or steric stabilization.
Because of their relatively low molecular weights, Mn generally below 5000 g/mol and Mw generally below 30,000 g/mol, and by reason of their balanced hydrophilic/lipophilic character, the secondary dispersions are extremely suitable for effecting a stable emulsification of both hydrophilic and hydrophobic non-self dispersing polyisocyanates in an aqueous environment and at the same time for acting as reactive components (cf. EP-A 0 358 979). The relatively low molecular weights of the secondary polyol dispersions means that such aqueous 2-component PU systems are generally not sufficiently suitable, however, in terms of their physical surface drying on wood as a substrate. The drying times (touch and dust drying) are too long and are unsuitable for industrial painting.
Examples containing primary dispersions as polyol components are generally more suitable as physically quick drying aqueous 2-component PU systems. These generally display molecular weight values of Mn significantly higher than 5000 g/mol and Mw values of generally above 30,000 g/mol. These primary dispersions can generally only be combined with (partially) hydrophilically modified polyisocyanates; a relatively uncomplicated dispersion can be performed with reasonably simple mixing equipment or by hand with a glass rod (as described in EP-A 0 557 844). If, for example, polyisocyanates hydrophilcally modified with polyether groups are then used as crosslinking agents and such coatings are applied to wood as a substrate, then paint films are obtained after drying which retain lasting discoloration under the action of color-imparting substances such as e.g. red wine, coffee, tea, mustard, etc., which on furniture in particular can lead to permanent staining. Such coating systems also display slight haze. If, alternatively, polyisocyanates containing external or internal anionic groups are used, the stain resistance improves but the level of resistance is still not satisfactory. Moreover, the mutual compatibility of the components in such systems deteriorates, and the films exhibit increased haze.
It was an object of the present invention to provide polyol dispersions as components for 2-component PU systems which form stain-resistant, haze-free coating films with polyisocyanate components containing anionic groups.
This object could be achieved by combining aqueous polyol dispersions, preferably of the primary dispersion type produced in the presence of special non-ionic surfactants, with polyisocyanates displaying both a hydrophobic character and a non-ionically or anionically modified hydrophilic character. This fact was all the more surprising because, as has already been mentioned above, polyol dispersions without such special non-ionic surfactants in combination with non-ionically modified hydrophilic polyisocyanates lead to paints with unsatisfactory stability values in respect of aqueous color-imparting liquids, and further addition of non-ionic surfactants to the 2-component PU system would have been expected to result in a further marked deterioration in resistance values.
The invention relates to an aqueous 2-component polyurethane (PU) coating composition containing
a) a vinyl polymer polyol dispersion, having a hydroxyl value between 10 and 264 mg KOH/g solid resin, an acid value (calculated from the sum of neutralized and non-neutralized acid groups) between 0 and 55 mg KOH/g solid resin, a number-average molecular weight Mn of at least 5000 g/mol and a weight average molecular weight Mw of at least 30,000 g/mol, a glass transition temperature of at least 20xc2x0 C. and an average particle diameter of no greater than 300 nm, which is prepared in the presence of
a1) 0.1 to 10.0 wt. %, relative to the sum of the solids contents of polymer polyol and polyisocyanate, of a non-ionic polyether surfactant,
and
b) a polyisocyanate having a viscosity at 23xc2x0 C. of no more than 12,000 mPa*s and either a non-hydrophilic or a non-ionically or anionically hydrophilic character, and
whereby
the equivalent ratio of isocyanate groups of component b) to hydroxyl groups of component a) is 0.2:1 to 5:1.
The invention also relates to a process for the preparation of such aqueous 2-component PU systems.
Component a) is preferably produced in the presence of
a1) 1.0 to 8.0 wt. % of the polyether-type non-ionic surfactant. Block polymers of ethylene oxide and propylene oxide according to formula (I) 
xe2x80x83and/or block and random ethylene oxide/propylene oxide copolymers based on fatty alcohols (II), and/or block polymers of the type according to formula (III), which can be obtained by polycondensation of propylene oxide and ethylene oxide on ethylene diamine, 
xe2x80x83and/or adducts of polyethylene oxide on fatty alcohols (IV) and/or polyether/polyester block polymers (V), are preferably produced or used as surfactant components a1).
Surfactant classes (I), (II), (III) and (IV) are particularly preferred as surfactant components a1). These non-ionic surfactants generally display a polyethylene oxide content of 5 to 80 wt. %, molecular weights up to 10,000 g/mol and hydroxyl contents of between 0.3 and 8.0 wt. %.
Component a) is produced in such a way that the non-ionic surfactant a1) is added either before or during polymerization. The introduction of a1) into the polymer batch prior to the production of a) is preferred.
The polyisocyanate component b) can display either
b1) a non-hydrophilic character or
b2) a non-ionically modified hydrophilic character and/or alternatively
b3) an anionically modified hydrophilic character.
If a non-hydrophilic polyisocyanate component b1) that is insoluble or non-dispersible in water is used in the aqueous 2-component PU system, a good dispersing effect can only be achieved using a highly efficient dispersing unit, e.g. a jet dispersing unit according to EP-A 0 685 544. Only in this way can the maximum possible chemical resistance values of the 2-component PU system be achieved after application of the film.
If, however, the polyisocyanate component b) is to be dispersed with the polyol component effectively and in a fine-particle manner using simpler dispersing units, such as e.g. an agitator, or using a 2-component plant with pre-atomising nozzle or possibly even manually, a non-ionic or an anionic hydrophilization of b) (as component b2) or b3)) is absolutely essential.
The non-ionic hydrophilisation of the polyisocyanate component b) is generally performed by modifying an unmodified hydrophobic polyisocyanate with a polyether monoalcohol, e.g. according to EP-A 0 540 985 and EP-A 0 959 087.
The anionic hydrophilization of component b) is preferably performed by adding 0.2 to 5.0 wt. % (relative to the sum of the solids contents of polymer polyol and polyisocyanate) of surfactants containing sulfate, sulfonate or phosphate groups to the unmodified, hydrophobic polyisocyanate component b). The particularly preferred, phosphate group-containing surfactants exhibit the following structures: 
Although the sole use of phosphate-containing surfactants having the above structure in aqueous 2-component PU systems is already known (e.g. WO 98/38196 and WO 98/38231), mixing the polyisocyanate component b) with the polyol component a), which has not been prepared in the presence of the non-ionic surfactant component a1), leads to unusable results: the polyisocyanates b) are substantially more difficult to incorporate into the polyol a), resulting in relatively unstable 2-component dispersions, and the dry paint films are significantly hazier. As demonstrated in the examples; the stability of the dry 2-component film on wood as; a substrate, for example, with respect to aqueous chemicals such as e.g. color-imparting substances (red wine, coffee, tea and mustard) is also significantly reduced.
The anionic hydrophilization of component b) can also be performed by means of sulfate or sulfonate groups as an alternative to phosphate groups. The use of sulfonate groups, which are present in the polyisocyanate in chemically bonded form, is preferred here. These can be produced for example by reacting polyisocyanates with sulfonate group-containing surfactants, which additionally also carry at least one group that is reactive in respect of NCO groups. Reaction products of polyisocyanates with 3-(cyclohexylamino)propane sulfonic acid are an example. The sulfonate groups can be present in quantities of 0.1 to 5.0 wt. % relative to the polyisocyanate.
The use of mixed hydrophilically modified polyisocyanates, such as described e.g. in EP-A 0 510 438, is also possible in principle. This involves simultaneous hydrophilization with non-ionic polyether groups and (potentially) anionic carboxyl groups.
The polymers a) are hydroxy-functional copolymers in the hydroxyl value range from 10 to 264 mg KOH/g solid resin, the acid value range from 0 to 55 mg KOH/g solid resin, which also display a content of chemically bonded carboxylate and/or sulfonate groups of a total of 0 to 97 milliequivalents (meq) per 100 g solids. The acid value refers here both to the free, unneutralized acid groups, particularly carboxyl groups, and the acid groups present in neutralized form, particularly carboxylate groups. The copolymers generally exhibit a molecular weight Mn of 5000 to 300,000, preferably 10,000 to 200,000, and a molecular weight Mw of 30,000 to 2,000,000, preferably 40,000 to 500,000 g/mol, determined by the method of gel permeation chromatography using polystyrene as standard.
The copolymers a) preferably contain
A) 0 to 7, preferably 1 to 5 wt. % acrylic acid and/or methacrylic acid,
B) 0 to 75 wt. % methyl methacrylate,
C) 0 to 75 wt. % styrene, whereby the sum of B+C is 10 to 85 wt. %.
D) 3 to 40 wt. % of one or more C1-8 alkyl acrylates or C2-8 alkyl methacrylates,
E) 2 to 74 wt. % of one or more monohydroxy-functional alkyl acrylates or alkyl methacrylates
F) 0 to 15 wt. % of other olefinically unsaturated monomers, whereby the sum of A) to F) is 100 wt. %, whereby moreover 5 to 100% of the acid groups incorporated by polymerization have been neutralized with aliphatic, amines or with ammonia, such that the content of anionic, salt-like groups in the copolymers corresponds to the figures given above.
The unsaturated acids A) and optionally F) incorporated by polymerization are, as stated, at least partially neutralized, such that the resulting anionic groups ensure or at least facilitate the solubility or dispersibility of the copolymers in water. If only low concentrations of salt-like groups are present, the solubility or dispersibility of the copolymers in water can be improved by the use of external emulsifiers. In all cases the water dilutability of the copolymers either as a dispersion or as a colloidally to molecularly disperse xe2x80x9csolutionxe2x80x9d must be assured.
The monomers B) and C) can be varied such that 10 to 85 wt. % of exclusively one of the monomers is contained in the sum of B)+C), whereby styrene is preferred.
Examples of C1-8 alkyl acrylates D) include methyl acrylate, ethyl acrylate, n-propyl acrylate, iso-propyl acrylate, n-butyl acrylate, iso-butyl acrylate, n-hexyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate. n-butyl acrylate., n-hexyl acrylate, 2-ethylhexyl acrylate, particularly n-butyl and/or 2-ethylhexyl acrylate, are preferred.
Examples of C2-8 alkyl methacrylates D) include ethyl methacrylate, n-butyl methacrylate and/or 2-ethylhexyl methacrylate.
Examples of hydroxy-functional (meth)acrylates E) include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxyisopropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate or any combinations of these monomers. 2-hydroxyethyl methacrylate and the technical blend of 2-hydroxypropyl and 2-hydroxyisopropyl methacrylate, generally known as hydroxypropyl methacrylate, are preferred.
The further monomer units F) can be substituted styrene derivatives, such as e.g. the isomeric vinyl toluenes, xcex1-methyl styrene, propenyl benzene, C5-C12 cycloalkyl (meth)acrylates, isobornyl (meth)acrylate, vinyl esters such as vinyl acetate, propionate or versatate, vinyl sulfonic acid, whereby the total amount of polymerizable acids (carboxylic acids A) plus optionally the acids cited under F) does not exceed 7 wt. %.
Ammonia or aliphatic amines such as e.g. triethylamine, dimethyl ethanolamine, diethyl ethanolamine, triethanolamine or any other aliphatic amines, preferably from the molecular weight range 31 to 200, can be used for the at least partial neutralization of the acid groups incorporated by polymerization.
The copolymers a) can be prepared by solution polymerization in organic solvents. Examples of suitable solvents include toluene, xylene, technical blends of alkyl aromatics (Solvesso 100, Solvesso 150, etc.), chlorobenzene, ethyl or butyl acetate, methyl or ethyl glycol acetate, methoxypropyl acetate, methoxybutyl acetate, butyl glycol, dioxan, ethylene glycol monoethyl or diethyl ether, dipropylene glycol dimethyl ether, acetone, butanone, methylene chloride or any mixtures of such solvents.
If the copolymers a) are prepared in solution, such solvents are generally used in such low concentrations that they do not need to be removed after polymerization and conversion to the aqueous phase.
Examples of suitable polymerization initiators for this radical solution polymerization include aliphatic azo compounds such as azoisobutyronitrile (AIBN) or peroxidic compounds, such as benzoyl peroxide, tert.-butyl peroctoate, tert.-butyl perpivalate, tert.-butyl perbenzoate or di-tert.-butyl peroxide.
Examples of molecular weight regulators include above all sulfur compounds, such as e.g. dodecyl mercaptan (dodecanethiol) or thioglycol.
On completion of polymerization, the solutions or dispersions a) are preferably prepared by adding neutralization amine directly to the organic polymer solution and then introducing it into the aqueous phase or by metering the organic polymer solution into the water phase, to which neutralization agent has been added, and homogenizing it. The organic solvent present during polymerization can then be partially or completely removed by distillation if necessary.
Production of the copolymers a) directly in the aqueous dispersion by the emulsion polymerization method is particularly advantageous, however. Peroxodisulfates, e.g. potassium or ammonium peroxodisulfate, are particularly suitable here as radical initiators. If the copolymers a) are produced by the principle of emulsion polymerization, external emulsifiers such as anionic emulsifiers, for example, such as those based on alkyl sulfates, alkylaryl sulfonates, alkylphenol polyether sulfates as specified in Houben-Weyl, Methoden der organischen Chemie, Erweiterungs- und Folgebxc3xa4nde, 4th edition, volume E 20, 1987 (part 1, pages 259 to 262), for example, or alkyl polyether sulfates, or non-ionic emulsifiers such as e.g. the alkoxylation and particularly the ethoxylation products of alkanols, phenols or fatty acids, can be used, which remain in the system following production of the copolymers and can be regarded as auxiliary substances c). Where such emulsifiers are present, a very slight neutralization of the acid groups present is often sufficient to ensure the homogeneity of the solutions or dispersions a). The neutralizing agents, which are nevertheless used at least in small quantities, can be incorporated into the system as early as the emulsion polymerization stage. These dispersions generally exhibit solids contents of 20 to 60 wt. %, a pH from 6 to 10, a viscosity from 10 to 5000 mPas and an average particle diameter from 50 to 300 nm (measured using laser correlation spectroscopy). The glass transition temperatures (measured by means of DSC) of the solid resins are above 20xc2x0 C., preferably above 40xc2x0 C.
Polyisocyanates suitable as component b) are in particular the so-called xe2x80x9clacquer polyisocyanatesxe2x80x9d with aromatically or (cyclo)aliphatically bonded isocyanate groups, whereby (cyclo)aliphatic polyisocyanates are particularly preferred.
xe2x80x9cLacquer polyisocyanatesxe2x80x9d based on hexamethylene diisocyanate or 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (IPDI) and/or bis(isocyanatocyclohexyl)methane, for example, are very suitable, particularly those based exclusively on hexamethylene diisocyanate. xe2x80x9cLacquer polyisocyanatesxe2x80x9d based on these diisocyanates are to be interpreted as the known derivatives of these diisocyanates containing biuret, urethane, uretdione, allophanate and/or isocyanurate groups, which after production have been freed from excess starting diisocyanate down to a residual content of less than 0.5 wt. % by known means, preferably by distillation. The preferred aliphatic polyisocyanates for use according to the invention include the hexamethylene diisocyanate-based polyisocyanates displaying biuret groups in accordance with the above criteria, such as can be obtained for example by the methods described in U.S. Pat. Nos. 3,124,605, 3,358,010, 3,903,126, 3,903,127 or 3,976,622 and consist of mixtures of N,Nxe2x80x2,Nxe2x80x3-tris-(6-isocyanatohexyl) biuret with secondary quantities of its higher homologues, as well as the cyclic trimers of hexamethylene diisocyanate corresponding to the stated criteria, such as can be obtained according to U.S. Pat. No. 4,324,879, and which consist substantially of N,Nxe2x80x2,Nxe2x80x3-tris-(6-isocyanatohexyl) isocyanurate mixed with secondary quantities of its higher homologues. Mixtures of hexamethylene diisocyanate-based polyisocyanates according to the stated criteria displaying uretdione and/or isocyanurate groups, such as are formed by catalytic oligomerisation of hexamethylene diisocyanate with the aid of trialkyl phosphines, are particularly preferred. The latter mixtures having a viscosity at 23xc2x0 C. of 50 to 500 mPas and an NCO functionality of between 2.2 and 5.0 are especially preferred. Monomeric polyisocyanates such as e.g. 4-isocyanatomethyl-1,8-octane diisocyanate can also be used, however.
The aromatic polyisocyanates, which are likewise suitable according to the invention but are less preferred, are particularly xe2x80x9clacquer polyisocyanatesxe2x80x9d based on 2,4-diisocyanatotoluene or technical blends thereof with 2,6-diisocyanatotoluene or based on 4,4xe2x80x2-diisocyanatodiphenyl methane or mixtures thereof with their isomers and/or higher homologues. Such aromatic lacquer polyisocyanates are for example the urethane group-displaying isocyanates such as are obtained by reacting excess quantities of 2,4-diisocyanatotoluene with polyhydric alcohols such as trimethylol propane, followed by removal of the excess of non-reacted diisocyanate by distillation. Further aromatic lacquer polyisocyanates include for example the trimers of the monomeric diisocyanates cited by way of example, i.e. the corresponding isocyanato-isocyanurates, which have likewise been freed from excess monomeric diisocyanates after production, preferably by distillation.
Unmodified polyisocyanates of the type mentioned by way of example can also be used in principle, provided that they correspond to the statements made in respect of viscosity.
The preferred use of hydrophilically modified polyisocyanates b) prepared by non-ionic or anionic hydrophilization of the above hydrophobic polyisocyanates is described above.
The polyisocyanate component b) can also consist of any mixture of the polyisocyanates cited by way of example.
The 2-component PU blends for use according to the invention are generally prepared by simply stirring the individual components a) and b) manually or by means of stirrers or in the case of poorly dispersible 2-component systems using a jet disperser, whereby NCO/OH equivalent ratios of 0.2:1 to 5:1, preferably 0.7:1 to 3:1, are obtained.
The non-ionic surfactant component a1) can in principle be added prior to or during polymerization of a). In the case of emulsion polymerization, if component a1) of the polyol component a) is incorporated prior to polymerization, it is added to the aqueous batch, generally together with an anionic emulsifier.
If component a1) is added during feed polymerization, it is generally added to the flow of monomer blend or, if it is sufficiently dispersible in water, to the aqueous flow of initiator solution. Surfactant component a1) can in principle also be added to the polyol component a) on completion of polymerization.
The compatibility between the polyisocyanate component b) and the polyol component a) and hence also the easier dispersibility of the aqueous 2-component PU system produced from these components is considerably increased if the polyisocyanate component has been hydrophilically modified either non-ionically or anionically. In some cases dispersion can then be performed by hand or using simple stirring devices such as e.g. an agitator.
Further auxiliary substances and additives c) known from paint technology can optionally be incorporated into the mixtures containing a) and b) for use according to the invention or into the individual components used in their production.
These include, for example, further quantities of water and/or solvent for the purpose of adjusting the appropriate processing viscosity of the coating compound according to the invention. The ready-to-use coating compounds according to the invention generally contain, relative to the total weight of all components, 2 to 25 wt. % of organic solvents and 10 to 75 wt. % of water. Further auxiliary substances and additives c) include, for example, pigments, fillers, flow control agents, thickeners, defoaming agents, deaerating agents and the like.
The coating compounds according to the invention can be applied by all conventional methods used in industry, such as e.g. spraying, immersion or brushing, onto any surface such as e.g. wood, metal, plastic or even mineral surfaces as substrate and dried at room temperature up to approx. 80xc2x0 C. Application on wood is particularly preferred.